Crabs are members of the Arthropod family, the largest group in the animal kingdom. Their distinguishing characteristic is their elaborate exoskeleton of armour. This provides the animal protection but creates a problem during growth when it must be shed and a new larger suit secreted. Many crabs mate when the female is still soft from moulting its carapace. Before moulting the female releases an attractant. Drawn to the female, the male approaches her and may wait some days before she moults. After discarding the old carapace, the crab expands by taking in water and stretching the usually wrinkled skin to a size larger than necessary. Lost appendages may even be replaced during this process. The deposition of a new shell takes 2 to 3 weeks.

Most Crabs can see very well. They respond to objects moving twice as minute a distance as can be detected by humans. Deep-water or cave-dwelling crabs have less acute eyesight; in these animals the senses of touch and vibration play the most important role in perception of their surroundings.